Go to the "search with in results" section and use the minus character then what ever collection of words you don't want to see. Use "" if it's a phrase.

I don't understand how people don't know this. I figured it out as soon as I hoped on AO3 and I've been telling fans much older and wiser than me about it for years now it seems.

*shrugs*

way simpler than a plug in. It's also how I find a lot of the fics I rec since I can get so exact in my search results.